It is known in the field of optical communications to utilize “chirp” to compensate for effects such as non-linearities. Chirping causes a signal's spectrum to spread and, therefore, reduces the power density. As a result, non-linear effects are also reduced. Chirping can also be used, for example, to compensate for dispersion caused when optical signals travel through optical fiber, as well as to compensate for other effects.
On prior art method is to use a fixed chirp which is inherent in a Mach-Zehnder-based (“MZ”-based) intensity modulator. Another prior art method is to use a separate phase modulator (“PM”) to implement chirp through phase modulation of the signal. The prior art, however, requires the PM to be synchronous. This is accomplished by driving the PM with the same clock source as drives the data modulator(s).
FIG. 1 illustrates one embodiment of a prior art implementation of a chirped wavelength division multiplexed optical communications transmitter 10. In that embodiment, a laser 12 and several modulators 14 are used to produce an optical signal for one of the wavelength division multiplexed channels. In this particular embodiment, the laser 12 produces an optical carrier, one modulator is used to modulate data onto an optical carrier in non-return to zero (“NRZ”) format, another modulator is used to amplitude modulate (“AM”) the NRZ format signal to produce a return to zero (“RZ”) format, and a third modulator is used to phase modulate (“PM”) the RZ format signal. The phase modulator introduces the chirp to the signal.
Each of the modulators 14 in the illustrated prior art embodiment are driven by a common or master clock signal generated by a master clock or oscillator 16 so that the operation of each of the modulators 14 is precisely synchronized. The synchronization of the modulators 14 is necessary in the prior art because the degree of chirp introduced by the phase modulator is a function of the relative phase between the data and the clock signals. This dependence on phase can be seen more clearly with reference to data points “1 mod” in FIG. 6, which is described in more detail hereinbelow. As a result, if the phase modulators of the prior art are not synchronized with the other modulators, the extent of the chirp cannot be controlled and the performance of the system will suffer. U.S. Pat. No. 5,526,162, issued to Bergano, illustrates the prior art approach in which a master clock is used to precisely synchronize various modulators in an optical transmission system.
For example, if a local oscillator is used to drive the phase modulator, it may be unsynchronized, resulting in the chirp of the optical signal to vary over time. This can cause significant detrimental effects because the variation in chirp can vary significantly.
The use of synchronous chirp has certain disadvantages. For example, an optical transmitter which is initially designed to operate without chirp, cannot be easily or inexpensively upgraded to introduce chirp. That is because it is difficult and expensive to upgrade a produce with a synchronous phase modulator. As a result, using synchronous chirp according to the prior art limits the ability to modify products in light of new or changing business environments.
Therefore, there is a need for systems, apparatuses, and methods which allow for signals to be chirped without the requirements and limitations of the prior art.